In recent years, therapies for malignant tumor using a gene delivery carrier have been studied in various ways. For example, with regard to an anaerobic bacterium Clostridium, a method of transporting a gene to a tumor site using a transformed bacterium has been proposed (see e.g., Patent Documents 1 and 2).
Likewise, with regard to an anaerobic bacterium Bifidobacterium, its transformant has been expected to be applied as a vector for probiotics or a vector for an oral vaccine for infectious disease (see e.g., Patent Document 3). Moreover, with regard to Bifidobacterium longum, its application to treatment of solid tumor has been suggested, because this bacterium accumulates in hypoxic solid tumor after systemic administration (see e.g., Non-Patent Documents 1 and 2).
Moreover, the present inventors have confirmed that, by transformation with a recombinant plasmid pBLES100-S-eCD carrying Escherichia coli coda fused with a promoter of a histone-like DNA-binding protein derived from Bifidobacterium longum, cytosine deaminase (EC3.5.4.1; hereinafter, referred to as “CD”) which is an enzyme that converts 5-fluorocytosine (hereinafter, referred to as “5-FC”) as a prodrug (precursor) of 5-fluorouracil (hereinafter, referred to as “5-FU”) having an antitumor activity, to 5-fluorouracil, can be expressed in a recombinant microorganism, and have reported that the recombinant microorganism, for example, recombinant Bifidobacterium longum, can be expected to be applied to enzyme-prodrug therapies (see e.g., Patent Document 4 and Non-Patent Documents 3 and 4). For applying this CD-expressing genetically modified microorganism to enzyme-prodrug therapies, it is further required that the microorganism should have a resistance to 5-FU at a concentration at least effective for an antitumor activity, which is converted from 5-FC by CD. Therefore, the present inventors have developed and reported a method of constructing such a CD-expressing 5-FU-resistant bacterium (see e.g., Patent Document 5). On the other hand, with regard to this CD, Non-Patent Document 5 has reported that when a mutation that substituted aspartic acid by alanine at the 314th (corresponding to the 315th position in SEQ ID NO: 28 of the present invention) amino acid of CD was added to CD-encoding DNA in Escherichia coli, mutant CD had a CD activity of converting 5-FC to 5-FU that was approximately 2.2 times (50/23) higher than that of wild-type CD before mutation (see the middle section of Table 1 in Non-Patent Document 5).
Various transformation methods have been reported on recombinant bacteria used in treatment of malignant tumor. In the documents described above, their respective methods of preparing a transformed bacterium have been reported.
For example, Patent Document 3 has reported a transformation method comprising the steps of: producing a shuttle plasmid that is mutually replicated in both Bifidobacterium species and Escherichia coli, using a Bifidobacterium species-derived plasmid and an Escherichia coli-derived plasmid; and producing a recombinant vector by ligating a gene of interest encoding a protein of interest to the shuttle plasmid, wherein the Bifidobacterium species used in the production of the shuttle plasmid is used as a host cell to be transformed with the produced recombinant vector.
In addition, for example, with regard to a method of preparing a shuttle plasmid pBLES100 used in the construction of the recombinant plasmid pBLES100-S-eCD, a preparation method comprising constructing this shuttle plasmid from pTB6 of Bifidobacterium longum BK51 and pBR322 of Escherichia coli has been reported (see e.g., Non-Patent Document 6).
Furthermore, methods of preparing plasmids pAV001 and pBRASTA101 have been proposed, which are capable of transforming Bifidobacterium longum with 100 times or higher efficiency than that of the shuttle plasmid pBLES100 (see e.g., Non-Patent Document 7).
Thus, various methods of constructing a gene delivery carrier useful for treatment of malignant tumor have been reported. However, all of these methods have specified microorganisms, plasmids, or the like used in transformation and have used usual transformation techniques as a transformation method itself. Moreover, these methods are intended to construct a transformed microorganism itself capable of expressing, in a target affected area, a gene of interest, for example, a gene for expressing a protein having an antitumor activity or a protein having an activity of converting an antitumor substance precursor to an antitumor substance. No documents has reported so far a construction method that is intended to improve the activity or expression efficiency of a protein itself expressed from the gene of interest.
Patent Document 1: U.S. Pat. No. 6,416,754
Patent Document 2: U.S. Pat. No. 6,652,849
Patent Document 3: National Publication of International Patent Application No. 2004-519236
Patent Document 4: Japanese Patent Laid-Open No. 2002-97144
Patent Document 5: WO 2006/109619
Non-Patent Document 1: Yazawa et al. Cancer Gene Ther., 7, 269-274 (2000)
Non-Patent Document 2: Yazawa et al. Breast Cancer Res. Treat., 66, 165-170 (2001)
Non-Patent Document 3: Nakamura et al., Biosci. Biotechnol. Biochem., 66, 2362-2366 (2002)
Non-Patent Document 4: Fujimori et al., Curr. Opin. Drug Discov. Devel., 5, 200-203 (2002)
Non-Patent Document 5: Sheri et al., Protein Engineering, Design and Selection, 17 (8): 625-633 (2004)
Non-Patent Document 6: Matsumura et al., Biosci. Biotechnol. Biochem., 61, 1211-1212 (1997)
Non-Patent Document 7: Tanaka et al., Biosci Biotechnol Biochem.; 69 (2): 422-425 (2005)